Thermal nuclear reactor



16 Sheets-Sheet 1 F. W. FENNING ET AL THERMAL NUCLEAR REACTOR 3 o so@ l 25 sept. 24, 1957 Filed March 14, 1952 mw e A/e, /Vf Inven fors sept. 24, 1957 Filed` March 14, 1952 SINN ,.O.U L .mv-l mm Q, Wig@ .mi Q .vdi mw of Inventar A forn ey Sept. 24, 1957 F. w. Fr-:NNING ETAL THERMAL NUCLEAR REACTOR 16 Sheets-Sheet 5 Filed March 14, 1952 FIG.'7.

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l THERMAL NUCLEAR REACTOR Y Filed March 14, 1952 16 Sheets-Sheet 5 FEGJQ.

A ttor/1 ey Sept. 24, 1957 F. W. FENNING 'ETAI- THERMAL NUCLEAR REACTOR 16 Sheets-SheeI 6 Filed March 14, 1952 HGJB inve/:fors

AttorneyV Sept.. 24, 1957 Filed March 14, 1952 F. w. FENNINGET AL THERML NUCLEAR REACTOR UNLOAU FACE 16 Sheets-Sheet 7' www By Sept. 24, 1957 F. w. FENNING ET A1.4 @2,8075

- THERMAL NUCLEAR REACTOR A Filed Maren 14, i952 man. UNLOAD FACE I wen fors AB y A Horn @y 16 sheets-sheet E Sept. 24, 1957 F. w. FENNING ET AL THERMAL NUCLEAR REACTOR 16 Sheets-Sheet 9 Filed March 14, 1952 RecrmG con:

I REMQVABLE FLOOR OVER B'IO. LAB.

In ven fons Affrney Sept. 24, 1957 Filed. Mach 14, 1952 F. W. FENNING FAL THERMAL NUCLEAR REACTOR 16 Sheets-Sheet l0 CONTROL FACE HOLEs IN GRAPHITE CODE SYMBOL DESCRIPTION N CROss LENGTH FROM GRAPHITE LETTER OF HOLE OF SECTION EXPERIMENTAL FACE EI I2 1" f EXPERIMENTAL 3f" THRO FROM EXPERIMENTAL E2 II 37/9 FACE I THRO'FROM EXPERIMENTAL AI 4 '4/2 g LARGE ANIMAL x FCE A2 |4I/2 3 (To REACTING CORE) A3 El SMALL ANIMAL I Ixix-M31 3' (TO REACTING CORE) ICI E 4 4'2'/2"(TO REACTING CODE) VION CHAMBER E l H IC2 [il s 55/2 (To REACTING CODE) 5 Of 'I'SABW" CORE CI-Cs CONTROL ROD I O /Q EXPERIMENTAL FACE) MORTUARY HOLES-SEE EXPERIMENTAL FACE EI es EEE DED D D l' 4 1 I l [TT- '--TTTTT-T-- l l' I l m f a Iz Iz Iz u I f U Il I Il I u1 l n l l l E I n n Q n A i 9( L ICs-i4 I2 o l l l o a l: I 2 I 'EJ-A a-EI l I o I l' a g m l Iz l' g d l I DI DI :PI m FLOOR I I I I LEVEL I Q2 lQ2 Q2 IQ2 Q2 f l' l L I I I l A A A A L I L -LLJ I F G. I9.

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F. w. FENNING ET AL Filed March 14, 1952 16 Sheets-Sheet l1 LOAD AND uNLOAD PACI-:s

HOLEs IN GRAPHITE CODE DESCRIPTION N CPOss LETTER OE HOLE OF SECTION REMARKS COOLING CHANNELS FlGgd 5| wITHIN PEACTING E79 CORE APPROX. FIGQb SEE ,G 9 COOLING CHANNELS 52 OuTsIDE PEACTING 885 I-'IGgb CORE APPPOx AI: AIRFLOW sLOTs 4 QL FACE TO FACE AI grb AO''gET To 5 ll'xIB/l uNLOAD FACE ONLY AEI COOLING AIP OUTLET 5 PPOM CONTROL PODs A52 AIP ESCAPE FROM B EXPERMNTL HOLES E|.E6, Iu A54 EEAIFTEASSEETE 4 X3 LOAD FACE ONLY AES AIP ESCAPE FROM 6 f4 sHuT OFF PODs AEG AIR ESCAPE FROM l 4 EXPERIMENTAL HOLES E REMOVABLE CORE 2 3522 DEEP xaEwIDE I-'ACE TO I=ACEl AEs M AEI CUNLOAD FACE) m AEs MRQW By [nvenfans A Harney 16 sheets-sheet 12 OM TOP IN GRAPHrrE GRAPHITE FACE Is'a I33 sTEPPED HOLEs 43 MIN DIA Ifal THROUGH SHIELD GRAPHITE MEAsuRING e MIN DIA'IVz" ONLY wITH PLuGs RIEFLECTORv /f REACTING CORE 'DIA 4"DIA.

4DIA.

REMARKS MIN DIA 4/z NQ CROss SECTION DEPTH ER OF OFF TOP FACE HOLES IN GRAPHITE OTHER HOLES F. wl FENNING ET AL THERMAL NUCLEAR REACTOP.

DESCRIPTION OF HOLE THERMAL Expl EXPERIMENTAL sACK HOLEs sHuT OI=I ROD DESCRIPTION OF HOLE PERIsCOPE `RI/ROMETER I 0N) [nven fors A Horn ey `SYMBOL SYMBOL CODE LETTER CODE LETTER Sept. 24, 1957 Filed MaIICh 14, 1952 CRANE RAILS Sep. 24, i957 F. W. FENNING ETAL THERMAL NUCLEAR REACTOR Filed March 14. 1952 `1e sheets-sheet 1s FMG. a2.

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A Horn ey Sem. 24, 1957 F. W. FENNI'NG ET AL 2,807,580

THERMAL NUCLEAR RECTOR Filed. March 14, 1952 -16 Sheets-Sheet 14 QQ .l

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Inventors Homey Sept 24, 1957 F. w. FENNING ET AL 2,807,580

THERMAL. NUCLEAR REAcToR Filed March 14, 1952 16 Sheets-Sheet l5 [nvenfor B VA Harney SQL Wi, W57 F. W. FENNING ETAL THERMAL NUCLEAR REACTOR Filed March 14, 1952 16 Sheets-Sheet 16 @gg I [nvenlons A Horn ey States Patent ffice 2,807,50 Patented Sept. 24, 1957 i 2,807,580 THERMAL NUCLEAR REACTOR Frederick William Fenning and Robert Flinders Jackson, Strand, London, England, assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application March 14, 1952, Serial No. 276,604

1 Claim. (CI. 204-193.2)

This invention relates to nuclear reactors of the graphite moderated air cooled type in which canned slugs or rods of fissile material are employed and is concerned inter alia with means associated with such piles for producing radioactive isotopes and useful heat under conditions of maximum safety.

To ensure such conditions it is necessary to control the level of radiation of all types over the working area round the pile and even over the environs of the establishment.

Fast and thermal neutron monitors and gamma radiation meters may be installed in known manner around the pile and working area, and connected to indicators and alarms in a control room.

Monitors may also be installed in the exhaust duct to check the activity of the eiuent air. There is an unavoidable and appreciable activity during normal operation but it might be considerably increased if say the sheath of a cartridge failed and permitted iission products to be carried olf by the air stream. '111e detection of a single burst cartridge, however, is difficult because of the high background activity and regular checks of the activity at the exit of each cartridge filled channel is desirable.

One particular feature of a nuclear reactor constructed in accordance with the present invention is the means provided for detecting the individual channel in which a fault condition has occurred.

Other features of the invention will be apparent from the following detailed description of a complete nuclear reactor embodying the invention.

In the drawings:

Fig. 1 is a diagrammatic vertical section of the reactor taken through a uranium channel.

Fig. 2 is a diagrammatic sectional plan. V

Fig. 3 is an enlarged sectional View of the uranium channel in Fig. 1.

Fig. 4 is a section on the line IV-IV of Fig. 3.

Fig. 5 is a section on the line V-V of Fig. 3.

Fig. 6 is a perspective view of the graphite mass.

Fig. 7 is an enlarged view of the part of Fig. 6 in the circle VII.

Fig. 8 is a sectional view through a typical experimental hole in the experimental face E.

Fig. 9 illustrates a plug used to close the hole shown in Fig. 8.

Fig. 10 is a block plan of the reactor showing the cooling system and handling gear.

Fig. 11 is a plan view of part of Fig. 10 showing further handling gear.

Fig. 12 is a perspective view, partly broken away, of the reactor from the control and load faces.

Fig. 13 is a plan View of the reactor.

Fig. 14 is a section on the line XIV- XIV of Fig. 13 (experimental face).

Fig. l5 is a section on the (control face).

Fig. 16 is a section on the line XVI-XVI of Fig. 13 (unload face).

Fig. 17 is a section on the line XVII-XVII of Fig. 13 (unload face).

Fig. 18 is a diagram of the experimental face.

line XV-XV of Fig. 13

Fig. 19 is a diagram of the control face.

Fig. 20 is a diagram of the load and unload faces.

Fig. 21 is a diagram of the top face.

Fig. 22 is a plan view of the unloading machine and hoist.

Fig. 23 is a vertical longitudinal section through the unload coin.

Fig. 24 is a plan view of the apparatus for detecting the presence of active dust in the air discharged from the channels in the pile.

Fig. 25 is a front elevation of one of the winding trolleys shown in Fig. 24.

Fig. 26 is a section on the line XXVI-XXVI of Fig. 25.

Fig. 27 is a side elevation of Fig. 25 in the direction of arrow B.

Fig. 28 is a side elevation of one of driving mechanisms, and

Fig. 29 is a section Fig. 28.

Referring to Figs. 1 and 2, the reactor comprises a 26 ft. cube 1 of graphite supported on a concrete base 2 and enclosed within a massive concrete shield 3. The four vertical faces of the cube are designated load L, unload U, charge C and experimental E as indicated in Fig. 2.

Control of the pile is exercised by control rods of high neutron capturing material which may be moved in and out of the pile. Shut off rods of a similar material are arranged to drop into the centre of the pile automatically and `stop the reaction in the event of any of several factors reaching a dangerous level. These rods are not shown in Figs. l and 2 but will be described hereinafter.

At the load and unload faces L and U spaces 4 are provided to contribute headers for cooling air which enters through a subterranean inlet duct 5 passes through a larger number of channels 6 (a typical one only is shown in Fig. l) in the graphite mass and discharges through the outlet duct 7.

The typical channel 6 is seen to be aligned with stepped holes 8 in the shield 3 and steel tubes 9 bridge the header 4 at the unload end to provide a channel along which uranium cartridges may be pushed from the exterior of the shield 3. At the load end V-section troughs 9a bridge the header 4.

Fig. 3 shows a typical uranium channel 6 to an enlarged scale with uranium cartridges 10 in position and Figs. 4 and 5 show the shape of the channel in cross section. The channel 6 is of reduced cross-section along its central portion to increase the air velocity and hence the heat transfer coefficient over those uranium cartridges 10 in the hot central region.

The uranium metal is in the form of machined rods 0.900 inch diameter x 12 inches long. These rods are encased in aluminium cans of wall thickness 0.0250.030 inch obtained by a deep drawing process. The open end of the can is closed by an aluminium disc rolled in with top of the can and brazed with a silicon-aluminium solder.

The cans are tested by subjecting to hot air at 300 C. for several hours and then to steam pressure, when any leak would allow water to enter the can where, at the temperature prevailing, it would form uranium oxide which is a power of greater volume than the uranium that it contains. Thus at any point where it is formed the aluminium case is deformed and a blister appears. This can be rejected.

The uranium metal must be almost free from impurity and the competitive absorption of neutrons should not exceed about 0.25. The average density is 19 gms/cc. and M. P. 1100 C. The iirst allotropic change on heating is at 650 C. when the volume increases by 0.3%, thus the slug centre temperature must not exceed this amount or splitting might occur. The aluminium sheath the control rod on the line XXIX-XXIX of l 1 must also be of highpurity. The melting point of alu- The V-shaped troughs allow air to discharge freely into.

the. Outlet header 4 andY the load hole & is. closed by' a plug 13. f

The graphite mass 1 .is shown diag-rainniatieally-,in perspectivein Eig. 6.

The reacting core ofY this mass may; Abe .regar-.dei as; aV

cylinder ltabout 2 0 ft-ihf diameter, and. 2.0l ft.; tous; Surrounded on all sides by a neutron reilecting region 15 about 3Y ft. thick. with additional masses. at the corners to complete the subs- Croslins channelseatliis. iysimilar tothe channels 6 are provided in the reecting region, the channels A6u being of thesutallerseetioni shown in Fig. throughout their length. s.

f Fromthisnroieet thegtouahd sidefthennat columns 16 and 17 respectively. The graphite is penetrated frornload to unload tace by anexos. 117.6.,0fof; channels@ arranged in@` square. lattice of side 7% inches. The length 71A inQhesQr 011S .12itCh GP).- iS. a basic-measurement; in; the construction ofthe graphite and. the basic; graphiteblock. from which the rnass is,V builtI is 7%; X 71% X 2,9 iIlQlzlQSl. e., 1 X 1 X 4 pitches This gives. rise to the construction shown in Fig. 7, vvhere.the channelsg6'are. seen to.b e formed byv blocks tyre A (odd layers), and blocks type B` (even laaters). In order to form ther many holes and, renjiovableY cores whichy as will `be hereinafter described-1 traYeISQ the pilefrom trop,I experimental and control faces, variations on the basic types are required. The cooling channels 6, are not interrupted `by any holes, traversing the pile from otherdirectiojns.y `In order that holes. in tl'regrlpltte.A

1 may lineup with holes. through the shielding 3f with the; greatest possible accuracy machining tolerances oi $00025 inch are necessary Von alll leading dimensions on the graphite blocks. M Y A The function of the. graphite is to slow down neutrons. Absorption of neutrons is, however, undesirable and, occordingly a'graphite of high density and high purityy is. required. One; part per million by weight of boron and. even less of some rare earth elements is unacceptably large.- One part per thousand of water is a serious irri.-y

purity High. density, high purity and: goodmechanical, properties are to someextent conflicting requirements .fQIf C9131` mercial graphite and a balance has been struck using rhaterial of density about 1.64. effectiveV absorption cross section per'atom of 4.9. millibarns (one b arn.=,l,0fV24 sq. cm.) and anV ultimate tensile strength of 0.5 tonper sq. inch). y

|Ihe shieldinground the pile. 1 and outlet air duct; 7;" is designed toreduce the neutron andl gamma activity;V in the pile building to a ligure negligible compared to the; tolerance level for an 8 hour shift. This ensures.L that a very low background is observed` on counting equipment usedv in the building for experimental purposes.

t Within the concrete shield'3 on allY sides is a thermal shield 18. of interlockingcast iron. plates 6" thick, which are hung from a steel frameworkcast in with lthe outer concrete v(biological) shield 3. This thermal shield 18 by virtue of its high capture cross section for thermal neua trons and high density, absorbsmost of the thermal neu,- tron and gamma flux incident upon it. Thus most of-'the total. heat released in the shielding (4 kw. at full power) occurs inthe thermal shield 18 where the heat is readily transferredI tothe coolingair. rlhe'neutrorl and gar-ninalluXes leaving the thermal shield 18 are. reduced` by a ,large factor but Vare still highV by biological standards. The. biological shieldv 3. of approximatelyv 61/2 feet of bany-t.

; a seal on this lower surface a key 22 consisting of 9 inchesV tes concrete serves to slow down fast neutrons, capture the thermal neutrons produced and absorbs the gamma rays arising from this neutron capture as Well as absorbing any of the two latter which escape from the thermal shield 18.-

The thermalV and bioshields .give protection from core radiation but there are many load, unload and, experi: mentak holes passing through theI shieldingall of which must also be made, proof; Iagainst the escape` of radiation. As shown inr Figs. 8. and 2 steppedy tubes 19 of stainless steel are cast in with the concrete .shield 3. to line one such hole 24 and into this tube tits a correspondingly stepped mild steel plug 20 lled.' with concrete. There are two steps, each of da inch to inch according to the size ofthe, plug, whilefthe clearance; between vthe-"plug` and the. tube varies. from 1A@ to bis inch. Thus an annular-A neutron and; gamma beam escaping d'ow'nithe clearance space. meets. an obstruction: atV each step. and-is-'scatter'ed The. hOIZQntal (square-..5ectiorr) experimental holes have The Stringer. 21, ts into a recess in the. end `of1the plug 20121114 isv secured; by; a, mild; steel dowel 21a. ST0, effect 0f Pala-Hill. 'and V3y inches of Steel'. is. inserted into the spaceformed byA messing at 25 thelower surface of both plus audrhole over alengthot; 1,2 inches, Y

The thermal columns 16 and,1 7;have lead lle'd mild steel doors Z5 apprmrirnatelyy 1.2, inches, thick, these may be removed in sections. Where; experimental. holes pass through the doors lead lled mild steel stepped: plugsare. provided as shown iuzFiss. 8v and. 9.

The, general arrangement of the cooling system is shown. in plan in Fig. l0. Coolingl air is drawn into a building 26 through louvres 27 and through lilter 28 along'fan inlet venturi 2.9, thence through the pile.. to an outlet veuturie 30, and to the exhaust fans 3.1. The. exhausters 3 1 discharge ur)- a 20.0, feet stackt the base of which isindicated at In this manner thewhole Pile structure is.v kept under suction andranyr leak is(` fresh air inwards and not possibly actiyated air outwards.v

There are, 5 main exhaustersl each driven by av 1400 inductionrnotor (not shown). Each exhauster has 1 a capacity of 66,000 C. F. M.; air at-80. C. with a pressure rise of 2.6 p. s. i. discharging to atmosphere. Only 4 of 'these exhausters are. requiredto operate the'pile at 6000 kw., when the discharge will be 2 6 4,000 C. F. M. at 80 C. (i. e. 180,000 C. 1F". M. at 20 C. at air intake at 60I C. tem 1i se across thepile). 'Ifhere` are in addition ltwo auxiliary exhausters'31a each driven by a V71/2 H. P. induction motor.y Eachhas a; capacity of 15,;000. C.V F. M. air at C. with, a pressurerist` of Irl/2y` inch. water discharging to atmosphere... vThese allXlgiary exhausters 31a, are provided; Y -Y (a) For cooling andV Ventilating the pile'during shut down periods.

(b) For coolingI immediate/ly. following armain'powerY failure when they obtain electrical'supply from batteries or a diesel generator located at 34.

(c) For operating the pile. at lowpower levels.

operate withoutthe main exhaustersat 9.00 kw. withv a temperature rise of 60 C. across the pile.

For the production of useful vheat a gilledtube type stream are irradiated in the passage throughV thepile and, may become active,warnd are later carriedaway bythe stackexhaust and possibly deposited over the countryside.. The most serious are the heavier particleswhichmightr Whenl the 'auxiliary exhausters are both running the pile will- Butterlly valves bel'oweach` aso'atsso be deposited fairly soon after exhaust and might build up an appreciable concentration on the ground and buildings in the vicinity of the site. Such heavy particles are not normally airborne initially and so are seldom carried into the cooling stream. Precautions are therefore taken to ensure lthat local conditions do not give rise to dust in the immediate neighourhood of theintake.

Dust particles may cause erosion Within the pile, parrtcularly of the relatively soft graphite. Besides the resultant mechanical deterioration, erosion increases the dust content of the exhaust gases but the eroded matter will be mainly graphite dust which does not carry a high specific activity.

Deposition of dust within the pile is not serious because the air velocities and turbulence are much higher than in the ambient air. There may be a certain amount of impact deposition, an accumulation of which could cause a deterioration of the pile.

Water vapour is an undesirable component in the cooling air because it is a serious labsorber of neutrons and an accelerator of corrosion. The amount normally present is not suicient to cause trouble; it will not accumulate in the pile because the temperatures therein are above ambient.

As an additional margin of safety against contamination of the environs .of the site, particularly important in the event of a burst or damaged cartridge which would lallow very active material to be carried off by the air stream, an outlet filter 33 is provided just before the main fan header below the battery room 34.

This lter removes all particles of microns or greater, lighter particles being almost continuously air-borne.

The inlet filter 28 comprises a standard commercial type filter incorporating throw-away cotton Wool pads. It consists of 204 sections, 2'6 x 2'3", each containing 10 pads. The frontal area is 'about 1100 sq. ft.; the pads are V-shaped and the effective area is therefore some 10 times greater. The air velocity through the filter is about 0.25 F. P. S. With such la large filter area the pressure drop is quite small; for an lair flow of 200,000 C. F. M. the initial drop is less than 0.1 in. water gauge rising to 0.375 in. Water gauge during operation, at which point the filter pads are replaced. The pads never become active and their disposal involves no hazard.

The space available for the outlet filter 33 is restricted to a frontal area of 262 sq. ft. This lter accumulates active dust, so it is convenient to have duplicate parallel units in one of which the active material can be left to decay before disposal while the other is in use. Cotton wool pads are used for the filter elements with the usual V-construction. The total effective area for each filter is about 1600 sq. ft. and the air velocity through the filter is nearly 2 F. P. S. Consequently the pressure drop is higher than for the inlet filter, ranging from 1 in. W. g. initially to 5 ins. W. g. before disposal and they require to be renewed more frequently.

The accumulated dust can be handled without undue hazard by leaving it on the filter for about one Week for the activity to decay. The pads can then be removed and, if found economic, can be cleaned and re-used by a reverse air-flow and agitation. The dust can be collected by a vacuum cleaner with a ne filter and handled as mildly active Waste.

The pressure drop through the main cooling channels of the pile is so considerable that no very special attempts to minimise duct losses are necessary. The ducts are rectangular with an average cross-section of 80 sq. ft. which for a flow of 180,000 C. F. M. gives an air velocity of about 35 F. P. S. The corresponding pressure drop over a total duct length 600 ft., including venturis and headers is estimated at 0.1 p. s. i., representing only `a few percent of the total drop of 2.6 p. s. i. p

`The duct is of normal reinforced concrete construction, waterproofed with a layer of bitumastic. On the inlet side noyspecial considerations arise except near the entrance to the main pile structure where its walls may be subjected to some gamma and neutron bombardment. To reduce the heat generated in the concrete in this area, the thermal shield 18 of the main pile is decreased to 3" thick and extended back into the duct for a distance of 23 from the pile face as indicated .at 18b in Figs. 1 and 14 to 17.

On the outlet side, more difficult conditions are encountered. The air temperature on leaving the pile is to 100 C. an has appreciable gamma activity due to the argon content and possibly to activated. dust particles. In addition there is some neutron activity close to the pile outlet.

The thermal shield 18 is again extended into the duct for a distance of 23 at the reduced thickness of 3 as also indicated by the reference 18b in Figs. l and 4 to 17 and from there a l sheet steel lining is provided right up to the fanhouse.

Some air (from the building) is drawn in behind the plates of the shield for cooling purposes.

The handling and other facilities are shown in Figs. 10 and 11. Below floor level at the experimental face E is a biological laboratory 35 served through a hole in the floor by 1% ton hoist 36.

Several experimental holes are accessible from this laboratory as will be described hereinafter.. Also at the experimental face E are two ten ton platforms 37 (Fig. 11) spanning the space between the face E and the wall of the building housing the reactor and moveable vertically and horizontally by means of overhead cranes to any position on the face. One ten ton removable platform 38 is provided at the control face above the level of the control rod and ion chamber gear 39 [Fig. 10) described hereinafter. Ten ton overhead travelling cranes 40 and 41 serve the experimental and control faces respectively over the whole length of the building and a ten ton crane 42 serves the unload face. The cranes 40-42 operate at the level of the top of the reactor. A five ton crane 43 serves the top face and load end at high level. Twelve ton hoists 44 and 45 also serve the load and unload faces respectively and a half ton hoist 46 serves the top face.

Reference is now directed to the perspective view of the reactor (Fig. 12) and to the subsequent elevations, sections and face diagrams (Figs. 13 to 21).

The weight to be carried by the base 2 (Figs. lll-17) is not very considerable. The weight of the graphite l is about 800 tons and the total for the pile including bridge tubes 9 (Fig. l5) lower thermal shield 18a etc. is of the order of 1000 tons. Spread over 26 ft. square, the loading is only about 2 tons per sq. ft.

Mild steel girders 47 are set in the upper surface of the base 2 which rises from the main apron 48. The base is hollow for economy reasons and is vented mainly to prevent the possibility of accumulation of dangerous fission products and also to remove the small amount of heat generated and conducted into the concrete. yOn the girders 47 are laid the 6" cast iron plates of the lower thermal shield 18a which are carefully leveled to act as the main floor on which the graphite is laid.

The main apron 48 carries the base 2 together with the shielding structure, hoist foundations, etc. The loading is again fairly light amounting to about 10,000 tons over an area 60 x 80 giving a pressure of 21/2 tons per sq. ft. These figures are quite conservative for the type of chalk subsoil existing at the site. The load is roughly balanced on the apron to avoid the possibility of tilting as the foundations settle.

The principal function of the thermal shield 18 is to reduce the y-ray intensity from the pile as quickly as possible and to dissipate the heat thus generated; a dense material of good conductivity is required. Iron is quite suitable and since no special strength properties are required, cast iron plates are used. The thickness is fixed p at 6" from shielding considerations and the complete atari,caso.t

interlocking jointsfso that no'straghi-through path. CX?V ists any direction, through which radiationbfr neutrons, can escape.. These opiates are individually supported on steel girdersV 49 castI in with the` concrete of, the main shield. f y

The thermal shield 18 is also quite an efficient absorber of slow neutrons. Such absorption, gives. rise too-radiation which will not be. satisfactorily stopped when produced near the outer surface ofA the, shield 1 8.l Itis not very effective in slowing down fast". neutrons by elastic collisions because ofthe high atomic, mass of iron, but it has a moderate cross-section for inelastic scatteringandl does Contribute to tbereduction ofthe number `of; fast neutrons leaving the pile.. Y

At least one face of the thermal Shield-18 is in` contact with; the cooling, air. stream and because of the. relatively high conductivity, "the heat generated` throughout the volume of, the shield is easilytransferred to the, coolant.

Even at -the outside of the thermal; shield, the neutron and -rayintensity are still very Amuch above biological tolerance.- Slightly dilerentconsiderations apply (in. this shield. Although kthere is. still consiiilertlblev -rayradiation to be stopped, particularly that emitted in the outer regions. of .the thermal shield due toneutron absorption there,l a' more important function is to absorb as completely as possible all thermal neutrons and to slow down any fast ones. For this reason. a material rich in-fhydrogen is desirable, for this element `combines an ability to. slow down fastl neutrons very rapidly because of its low atomic mass, with a high capture crosssection. High density is desirable to eliminate. -.radiation, which not only enters from the thermal shield 18 but is also produced in the biological `shield 3 by the neutrons absorbedy there.

A material giving a good compromise of these factors with economic considerations. isp concrete. The mix and method of placing -is-v chosen to give the highest possible density and this is. obtained by using barytes instead of normal aggregate. rlhe density attainable if the normal 'aggregate is Wholly replaced by barytes is 3.5. The density actually obtained in the shield 3 is 3.2L With flintaggregate the gure would be about 2.3 and its use would involve increasing the shield thickness by 3.2/ 2.3 or approximately 40%. n y

The two thermal columns 1-7 and 16 (Fig. i7) extend- H ing from the main. graphite block 1 through the shield 3,

one on the experimental side E of thepileV andthe other' at the top T consistof columns of the highest grade of graphite supported in cast-iron fram-ing 50 built into the shield. Their function is to provide a beam of slow neutrons at the shield face for experimental purposes.

A short spur of graphite projecting from the reflector region 15 (Fig. 6) abuts the columns, witha clearance of about half-an-.inch Any fastneutrons crossing into the thermal columns are slowed down; since the length of the columns (about 6 to 7 ft.) is many slowing down lengths there is negligible fast neutronrux at the column faces. The graphite in the columns is also quite effective inV attenuating the gamma flux. A 7" thick bismuth sheet 50a (Fig. l17) is provided at the inner face of the top column to givesadditional reduction of theigamma flux; bismuth is relatively transparent Vto'tlrlermaljneut-rons. It is not structurally feasible to provide asimilar bismuthrshield at the side column. v Y 7 v The inner faces of the cast-iron framing-5G are lined with 0.08 in. of cadmium to absorb slow neutrons and prevent their entry into the biological shield and through this to the working yarea of the pile building. The cadmium shieldingv gives some collimation tothe emergent neutron beam.

The faces of the columns are normally closed by cast iron shields. 25 filled withflead and faced internally with 0.708 in.` cadmium sheet. This. ensures that, there is no escape of neutrons or gamma, radiation into the building when the columns16 and 17 are. not in use.v YThese shields s l 25. can removed. in sections; or small areas over enperia; mental Ylyiioles in the shields can be opened up. when ex-i PCrmen-tal neutron beams are required.

' The slow neutron beam 1s attenuated alongthethermal .4 column so provision is made for the remoualli a portion of the columnswhen higher fluxes arerequiredi YAt the top column 16 the whole of the graphite can be remoyed;

reasonable protection from gamma'adia'tioll isfprovridd by the. bismuth'shield. 50a (Fig. (17),. Because there is no bSmU-,th shield en ;the'fside. column 17, the inner2 ft. of graphite 17a shown therein is made non-removable. Y

Control rods 'are` provided toV maintainA the effective multiplication constant of the pile at zero for steady opera ation, to increaseor vdecrease it'slightly when` it is necessaryto alter the power level and; to.. providean adjustment to compensate'for deterioration off the pile orfor the'in-j sertion of erperimental rnaterial. v v f Fourrmain control rods C1 to.C4 (Figs. 12,; 1.5, 16. andV '19), each capable of absorbing 0.3% ink and aline control rod'CS, with one tenth thisfrange. are arranged to. enteri thepile horizontally from the control faCet the mainrods C1 to, C4'Y being spacedwon a semi-circle. of radius 3'6"V from the centre line, with the vfine control rod C Sl offset at 86 from the centre as most clearly shown in Fig. l9.V Y Theneutron` absorbing material isV boron in the form of boroncarbide powder which is conveniently refractory material.. This is contained in al mild steeltube 2VO.'D.

and 25- ft.` long, of which they rstnl17 ft. isboron carbide and the remaining 8- ft. is. lead and concrete actingv as a shield against radiations streaming yalong the tube and entrance hole. The rods C1 to.- (,25v inevitably become Iactive and it is therefore necessary tofprovide an extended shield 5 2 (Figs. 12 and 16.) outsidel the pile enclosing the rods when fully withdrawn. Y Y Y The.v rods C1 to C5 are positioned to an accuracy of .1/10 mm., and the position is indicated ata control point to the saine accuracy; A 22 ft. extension, 208 to the rod is connected to amoving head 201 incorporating a D. C. motor 2,02 and pinion 203 driving on a fixed raclc204v with guide rails 205. This systeml requiresless space than. a fixed head` and moving rack, although it involves a little extra complication in cabling. The rack land pinion are cut to give the necessary positional accuracyl and a multiple gearedV dilferential'magslip device 206V driven by al pinion 20.7 engaging the rack is used to set and indicate the rodiposition; three dials operated from the magslip device and situated" in the control room enable this position to be read to the required accuracy over the full range of movement of 1000 crns. A rectifierv and Y a group, at fast or slow speed, by--push-button control;-

alternative handwheel control for each rod. is provided. All these arelocated at a main control desk in the control room (Fig. 10).. Interlocks, safety devcies, etc. of; conventional design are incorporated to prevent damage Y to the mechanism or to the pile.

Two independent banks of shut-oifrods SQ- are also provided each bank beingA capable of reducing k by .112%- and ensuring that the pile closes down. Sixprodsare used in each bank and all the rods in one. bank areY normally operated together; they enter the pile vertically from vthe-'- topV face and are spaced round a circle of about 6l ft.

radius from the centre of the face and lie just outside the top thermal column 16 as seen in Figs-.13 and 721,.

The timeconstant of thepilerrequiresthat the rods- SO,

can be fully inserted' in less than one second, including the time of operationof'alarmand actuating. mechanisms. i

The rods are injected into'the pileA by individual. directe acting pistons operating at 100 p; s.'i.arV pressure which is maintained, allf the time.' YTheyv are Vraisedfand heldunasainst this pressure by; air at 5.00p. s.; il... acting via a 

